1. Field of the Invention
This invention relates to a method for improving the adhesion of a photopolymerizable composition and the photopolymerized composition formed therefrom to copper. The photopolymerizable composition is useful as a dry film photoresist.
2. Description of the Related Art
Photosensitive compositions particularly useful as photoresists are well known in the prior art. The compositions can be positive-working or negative-working. Particularly useful are negative-working compositions, also known as photopolymerizable systems. The photopolymerizable composition generally contains a polymeric binder, at least one monomeric or oligomeric material capable of polymerizing and/or cross-linking, and a photoinitiator or photoinitiator system. Exposure to actinic radiation initiates the polymerization and/or crosslinking reactions, resulting in insolubilization of the material in developer solvents. The thus formed latent image can then be developed by treating with developer solvent. The photopolymerizable compositions are generally formed as a dry film on a support. The dry film is sandwiched between the support film and a coversheet and stored in roll form.
The photosensitive film is conventionally unwound from a roll and, after the coversheet is removed, laminated to a substrate. The laminated composition is exposed imagewise to actinic radiation, with the support film stripped from the photosensitive composition before or after the exposure step. The exposed film is then treated with developer solvent which removes the unexposed areas of the photosensitive composition to form a resist image of polymeric material. The thus uncovered portion of the substrate can be treated as desired, followed by removal of the polymeric resist image.
For the high resolution desired in the electronics industry today, it is necessary for the resist to be removed cleanly in the development step without leaving any residue on the substrate. The addition of low molecular weight acids to reduce scum and stain formation in aqueous processable photopolymerizable compositions has been disclosed in Covesleskie et al., U.S. Pat. No. 4,510,230. However, in addition to low residue and scum formation, it is also important that the photoresist have good adhesion to the substrate, usually copper. The board with imaged resist is treated with a variety of chemicals during the manufacturing steps, typically etching and/or plating chemistries. Inadequate adhesion of the photoresist to the substrate can result in the chemistries reacting underneath the resist in areas intended to be covered. Thus the quality of the end product is reduced and failures can occur. Photopolymerizable compositions containing benzotriazoles and benzotriazole carboxylic acids have been shown to improve the adhesion of the compositions to substrates in, for example, Hurley et al., U.S. Pat. No. 3,622,334; Weed, U.S. Pat. No. 4,680,249; and Tohda et al, U.S. Pat. No. 4,741,987.